Oh I think i might have just responded to your comment on youtube xD Ill repost here though just incase its seen first haha
Thank you thank you!
In regards to the issue you've mentioned, there might be a couple of things at play here.
Not sure if youtube allows links in comments but if there was any way you might be able to screen record you scene set up I can try and help!
If you are able to post an issue here with details that would be amazing:
https://github.com/CarbsCode/CarbsAsepriteExtensions/issues
The modify exsisting button generally is used if you've got some sort of animated set of cels, or even cels with data already in them. (Showing the dot filled in).
That being said how it should be working is it doesn't modify the first or last selected cel as those are considered keyframes in the tool. It only modifies the cels between them so that the your to and from cels stay put.
When you say second frame doesn't change do you mean the last cel or is it more so like you've got 5 cels selected with data and cel number 2 of the 5 isn't moving? Actually I might have run in to this while making the video when showing the tween interpolation the first 2 cels or frames were the same so this might be a bug that existed before the update even 😆
Something I need to look in to I guess haha